The present invention relates to methods and compositions used in filtration assaying of isotope-labelled particulate sample.
Many biological, and other, samples containing radioactive isotopes are counted by liquid scintillation counting. Samples may be dissolved in scintillant or, if insoluble and finely divided, may be suspended in scintillant gels. Particulate samples may also be filtered onto a filter support layer and this is often used where unincorporated isotope must be washed from the sample, for example from biological cells.
Semi-automated harvesting methods exist for this procedure but the samples must then be placed in liquid scintillant, usually in separate vials, before counting in a liquid scintillation counter. Recently, a new type of flat-bed scintillation counter has been developed whereby multiple samples deposited on a filter support are placed together in scintillant without being separated (See UK patent 1,586,966, U.S. Pat. No. 4,298,796). This allows a considerable reduction in preparation time and also in the amount of scintillant required. The scintillant however usually contains an organic solvent which poses problems of disposal even in small amounts. It is also a problem for some samples with small molecules which may dissolve in the liquid scintillant and thereafter diffuse away from the correct sample area thereby limiting the general use of the flat-bed scintillation counter.
There have been several attempts to solve these problems. One is to use a special sheet of solid scintillant which is heated until it melts and soaks into the filter. When the composite cools the scintillant solidifies and any further potential diffusion of the sample is halted.
As another solution, the filter itself may be composed of scintillant so that the close approximation of the sample particles to the filter gives rise to countable scintillations (UK patent 1,586,966, U.S. Pat. No. 4,298,796). A variant technique is that the filter may support solid scintillant particles (such as Yttrium silicate) on its surface, placed there by filtration, scattering or spraying onto appropriate adhesion means (Int. patent WO 89/02088), ready to receive the samples by filtration. However, for this and the previous method, about half the electrons emitted by the decaying atoms of isotope will pass straight out of the filter and therefore will not encounter the scintillant, with the result that the efficiency is thereby limited to a maximum of about 50%.
An improvement on this technique (Swedish patent 9901879-1, Int. patent PCT/GB89/00542) is to coat the fibers of the filter with a meltable scintillant such that, after filtration and drying, the filter may be heated so that the scintillant flows around the particles of sample and the counting efficiency is thereby considerably increased. Since the flow is on a microscopic scale there is no significant diffusion of soluble sample components away from the sample area. In practice, the solid meltable scintillant, the Yttrium silicate-filters and the meltable scintillant-coated filter all typically have a similar maximum counting efficiency of 70-80% compared with the best liquid scintillant methods.